N/A
The present invention relates generally to the field of optical transmission systems, and more specifically to a dense wavelength division multiplexed optical transmission system configured to provide per-band dispersion compensation with gap-free band structures.
In recent years, Dense Wavelength Division Multiplexed (DWDM) optical transmission systems have been increasingly deployed in optical networks. Although DWDM optical transmission systems have increased the speed and capacity of optical networks, the performance of such systems, especially those providing bit rates of 10 Gb/s or more, has traditionally been limited by various factors such as optical fiber dispersion and the non-linearity in an optical fiber""s refractive index, which can cause spectral broadening of optical pulses and degrade the transmission of high speed optical signals. Because such optical signal degradation tends to accumulate along transmission paths, fiber dispersion and non-linearity can significantly limit the transmission distance of high speed optical signals.
One approach to reducing the fiber dispersion limit and improving the performance of DWDM optical transmission systems is to install dispersion compensation fibers at intervals along a transmission path. For example, transmission fiber in the transmission path may have a positive dispersion shift that causes optical dispersion to accumulate along the path, and the installed dispersion compensation fibers may have a negative dispersion shift that allows the accumulated dispersion to return to zero or some nonzero value at a remote end of the transmission path.
However, this first approach has drawbacks in that both transmission fibers and dispersion compensation fibers generally have nonzero dispersion slopes. Different wavelengths included in a multi-wavelength optical signal may therefore be subject to different dispersion values in the transmission fibers and the dispersion compensation fibers. This can be problematic because the dispersion slope of a transmission fiber generally does not match that of a dispersion compensation fiber. As a result, although the accumulated dispersion may return to a desired value at the remote end of the transmission path for a particular wavelength, nonzero residual dispersion values detrimental to reliable optical transmissions may be evident for remaining wavelengths of the optical signal.
Moreover, in order to avoid a significant penalty or loss at the remote end of a transmission path, it is generally desirable to maintain residual dispersion values within a desired range of values. However, for high speed, multi-wavelength optical signals, the effects of fiber non-linearity, including self-phase modulation and/or cross-phase modulation, typically enhance the optical signal degradation caused by fiber dispersion, thereby making it difficult to keep residual dispersion within the desired range.
Another approach to reducing the fiber dispersion limit in DWDM optical transmission systems is to identify a plurality of spectral regions in multi-wavelength optical signals, and perform dispersion compensation on the respective spectral regions. Such an approach is employed in conventional xe2x80x9cRedxe2x80x9d-xe2x80x9cBluexe2x80x9d bi-directional transmission systems. However, this second approach also has drawbacks in that there is typically a gap between, e.g., the Red and Blue spectral regions that reduces the total channel count of the optical signal.
Still another approach is to filter multi-wavelength optical signals using conventional cascaded band filters before performing dispersion compensation on the filtered bands. However, gaps are generally required in this third approach to avoid a penalty or loss resulting from, e.g., isolation, polarization dependent loss, or polarization mode dispersion in the optical signal pass-band. As a result, this third approach also tends to reduce the total channel count of the multi-wavelength optical signal.
It would therefore be desirable to have a high speed DWDM optical transmission system that is capable of carrying multi-wavelength optical signals at bit rates up to 10 Gb/s or more. Such a DWDM optical transmission system would be configured to reduce a fiber dispersion limit without reducing the total channel count of a multi-wavelength optical signal.
In accordance with the present invention, a high speed DWDM optical transmission system is provided that reduces a fiber dispersion limit without reducing the total channel count of a multi-wavelength optical signal. The DWDM optical transmission system achieves such reduction in the fiber dispersion limit by employing a multi-wavelength optical signal comprising a gap-free band structure, and performing per-band dispersion compensation on the optical signal by adjusting residual dispersion values associated with one or more of the bands.
In one embodiment, per-band dispersion compensation of a multi-wavelength optical signal is provided by line terminating and/or regenerating apparatus at optical signal receiving sites of the DWDM optical transmission system. Each line terminating and/or regenerating apparatus comprises at least one band splitter, a plurality of fixed or tunable Dispersion Compensation Modules (DCM), and a plurality of optical de-multiplexors. A first DCM receives a multi-wavelength optical signal carried on a transmission fiber, and provides a dispersion-compensated multi-wavelength optical signal containing wavelength-dependent residual dispersion to the band splitter. The residual dispersion associated with the multi-wavelength optical signal is caused by the respective nonzero dispersion slopes of the transmission fiber and the first DCM. The band splitter separates the dispersion-compensated multi-wavelength optical signal into a plurality of bands such that no band gaps are formed between adjacent bands. In a preferred embodiment, the band splitter includes a 50/50 optical coupler configured to couple the optical signal to a first optical signal path and a second optical signal path, in which the first and second paths include respective pluralities of band filters. The plurality of band filters in the first path is configured to provide a first group of xe2x80x9coddxe2x80x9d bands, and the plurality of band filters in the second path is configured to provide a second group of xe2x80x9cevenxe2x80x9d bands. The band splitter provides each band in the odd and even groups of bands to a respective second DCM configured to reduce the residual dispersion associated with that band to a predetermined range of residual dispersion values. The second DCM""s provide the dispersion-compensated bands to respective optical de-multiplexors configured to separate the bands into their component wavelengths for subsequent processing.
Per-band dispersion compensation of a multi-wavelength optical signal is also provided at mid-points of optical signal paths by all-optical regenerating apparatus included in the DWDM optical transmission system. Each all-optical regenerating apparatus comprises at least one band splitter, a plurality of fixed or tunable Dispersion Compensation Modules (DCM), and at least one optical multiplexor. A first DCM receives a multi-wavelength optical signal carried on a transmission fiber, and provides a dispersion-compensated multi-wavelength optical signal containing wavelength-dependent residual dispersion to the band splitter. The band splitter separates the dispersion-compensated multi-wavelength optical signal into a plurality of odd and even groups of bands such that no band gaps are formed between adjacent bands. The band splitter provides each band in the odd and even groups of bands to a respective second DCM configured to reduce the residual dispersion associated with that band to a predetermined range of residual dispersion values. The second DCM""s provide the dispersion-compensated bands to at least one optical multiplexor configured to re-combine the bands to form a multi-wavelength optical signal for subsequent transmission. In a preferred embodiment, the optical multiplexor is configured to provide sufficient isolation to eliminate cross-talk between adjacent/non-adjacent bands.
In another embodiment, per-band dispersion compensation of a multi-wavelength optical signal is provided by transmitting apparatus included in the DWDM optical transmission system. Each transmitting apparatus comprises at least one band combiner, a plurality of optical multiplexors, and a plurality of DCM""s. Each optical multiplexor is configured to multiplex individual wavelengths into a respective band, and the respective bands are combined into a multi-wavelength optical signal by the band combiner. A respective DCM is disposed between each optical multiplexor and the band combiner to compensate dispersion contained in the respective bands.
In still another embodiment, per-band dispersion compensation of a multi-wavelength optical signal is provided by optical add/drop apparatus included in the DWDM optical transmission system. Each optical add/drop apparatus comprises at least one pre-amplifier, at least one post-amplifier, at least one first optical add/drop device configured to provide respective wavelengths that are dropped from the optical signal (xe2x80x9cdropped trafficxe2x80x9d), at least one second optical add/drop device configured to receive respective wavelengths that are to be added to the optical signal (xe2x80x9cadded trafficxe2x80x9d), and a plurality of fixed or tunable DCM""s. A first DCM is coupled to the input of the first optical add/drop device handling the dropped traffic, and a second DCM is coupled to the output of the second optical add/drop device handling the added traffic. The pre-amplifier receives a respective band of a multi-wavelength optical signal containing wavelength-dependent residual dispersion; and, provides an amplified representation of the band to the first DCM, which is configured to reduce the residual dispersion in the band to a predetermined range of residual dispersion values. The first DCM then provides the dispersion-compensated band to the first optical add/drop device, which provides the dropped traffic on a first optical signal path and remaining traffic to the second optical add/drop device. Next, the second optical add/drop device receives the added traffic on a second optical signal path, and provides a combination of the traffic provided by the first optical add/drop device and the added traffic to the second DCM. Like the first DCM, the second DCM is configured to reduce the residual dispersion in the combined traffic to a predetermined range of residual dispersion values. The second DCM then provides compensated traffic to the post-amplifier, which compensates for losses introduced by the first and second DCM""s and the first and second optical add/drop devices.
By providing per-band dispersion compensation with gap-free band structures at optical signal receiving sites, mid-points of optical signal paths, optical signal transmitting sites, and/or optical add/drop sites of a DWDM optical transmission system, fiber dispersion limits can be reduced or eliminated without reducing the total channel count of a multi-wavelength optical signal. In this way, transmission performances in high speed (i.e., 10 Gb/s or more) DWDM optical transmission systems can be improved.
Other features, functions, and aspects of the invention will be evident from the Detailed Description of the Invention that follows.